Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

FIGURE 23-8. The varioussources of exposure in a diagnosticx-ray room. A: Primaryradiation emanating fromthe focal spot. B: Scatteredradiation emanating fromthe patient. C: Leakage radiationemanating from the x-ray tube housing (other thanthe collimated primary radiation).tons are transmitted). The exposure due to leakage radiation is limited by federal regulationsto 100 mR/hr at 1 m from the tube housing when the x-ray tube is operatedat the maximum allowable continuous tube current (usually 3 to 5 mA) at the maximumrated kVp, which is typically 150 kVp.Factors AffectingProtective Barrier SpecificationsX-ray shielding is accomplished by interposing a highly attenuating barrier, usuallylead, between the source of radiation and the area to be protected in order to reducethe exposure to acceptable limits. The thickness of shielding needed to achieve thedesired attenuation depends on the shielding material selected. Lead is the mostcommonly used material because of its high-attenuation properties and relativelylow cost. Other shielding materials are also used, such as concrete, glass, leadedglass, leaded acrylic, and gypsum board.The amount of attenuation necessary depends on five major factors: (a) Thetotal radiation exposure level, XT, represents the exposure (expressed in mR/week) ata given location in an adjacent area, which is a function of the technique (kVp), thework load, the calculated primary, scatter, and leakage radiation levels, and their